It's very perplexing to me. Here are some examples:
https://pages.pomona.edu/~sg064747/travel/images/Turkey/Fortifications/1600-LandWallsArchGate.jpg
https://romeonrome.com/files/2013/09/66.EmperorsGoldenGate-600x450.jpg
https://pages.pomona.edu/~sg064747/travel/images/Turkey/Fortifications/1024-YedikuleGoldenGate.jpg
http://romeartlover.tripod.com/2a.jpg
http://www.livius.org/site/assets/files/21526/thumbnail_istanbul_wall_theodosius_02_ab.jpg
https://media-cdn.tripadvisor.com/media/photo-s/0a/0f/da/40/walls-of-constantinople.jpg
One theory I heard about is that it's due to cultural differences between the conquerors and the conquered.